Due to variations in manufacturing, a micromachined sensor needs to be calibrated with a known reference in order to work properly. For an accelerometer, it would be desirable to provide a known force/acceleration reference that is independent of manufacturing variations, and to calibrate the accelerometer by adjusting circuit gain so that an output signal indicates the force reference (force and acceleration are used here interchangeably because they differ only by a mass). Such circuit gain adjustment is typically done either by trimming a resistor with a laser, or by programming an on-chip EPROM.
As a practical matter, it is difficult to provide a force/acceleration reference. One acceleration reference that can be used is gravity. However, gravity has only one value and operates in only one direction. Using gravity as a reference would require rotating a wafer of sensors to different positions, thus necessitating new and complicated calibrating equipment.
Another method for providing a force reference is to shake the wafer or packaged sensor. Shaking a wafer is difficult in practice, however, because it too would require new equipment that would shake both the wafer and other calibrating equipment together. Calibration has been performed by shaking a packaged sensor and programming an EPROM formed on the die. This method requires extra fabrication to produce the EPROM, which then takes up space on the die; moreover, this method takes significant time to calibrate because each device must be shaken individually.
Due to these drawbacks, it is desirable to provide an improved method to calibrate a micromachined sensor to a known force/acceleration reference without shaking or rotating the wafer or packaged sensor.